Seres said ECOSPOR missed its primary endpoint of reducing the relative risk of CDI recurrence at up to 8 weeks compared to placebo.

CDI recurrence occurred in 44% of subjects (26 of 59) treated with SER-109—the company’s lead product candidate—compared to 53% of subjects (16 of 30) who received placebo.

The relative risk of CDI recurrence for the placebo population compared to the SER-109 population was not statistically significant, the company added.

One bright spot for SER-109: In subjects ≥65 years old, CDI recurrence occurred in 45% of subjects who received SER-109 (14 of 31), compared with 80% of those who received placebo (12 of 15). However, in subjects <65 years old, CDI recurrence occurred in 43% of subjects who received SER-109 (12 of 28) compared with just 27% of subjects who received placebo (4 of 15).

“The recurrence rates observed in the overall SER-109 treatment group, in the age-stratified subgroups, and in the placebo groups are inconsistent with our expectations,” Roger Pomerantz, M.D., Seres’ president, CEO, and chairman, said in a statement. “These are unexpected clinical results in view of the positive data in our prior investigator-sponsored Phase Ib trial, as well as in a wide range of supporting clinical and preclinical data.”

He said Seres’ priority will be to complete a full review of the clinical results and microbiome data of ECOSPOR, and compare it to data from the Phase Ib study: “Based on this information and pending discussions with the FDA, we plan to make any necessary changes to our development plans for SER-109.”

ECOSPOR enrolled 89 patients with multiply recurrent CDI, defined as three or more recent recurrences, in a randomized, double-blind, placebo-controlled, 24-week study conducted to evaluate the safety and efficacy of SER-109. Fifty-nine patients were treated with SER-109 and 30 with placebo. The study was conducted at 36 centers across the U.S.

Shares of Seres plunged 76% to $8.69 as of 10:27 a.m., from yesterday’s closing price of $35.77.